This paper includes an abridged
report originally presented at an ASAE conference and published in the
proceedings.As the report has been
used and or marketed improperly, causing much confusion, this paper will
endeavor to clarify various, important, aspects.

The report clearly identified the
methods utilized to evaluate a system prior to installing an aerobic system in
an attempt to recover a failing system. It is not possible however to recover
all failing systems using aerobic effluent.A comprehensive investigation and analysis of all information gathered
must be preformed prior to attempting a recovery.

This paper presents three case
studies where aerobic effluent was utilized to recover failing onsite
systems.The sites are described as
follows:1) a residential system with a
gravity drainfield, 2) a residential system with a mound, and 3) a café with a
gravity drainfield.The paragraphs to
follow summarize the methods employed when determining the cause of the
failures.This report also demonstrates
how the onsite systems can be recovered.As a comparative analysis the data associated with each case study is
tabulated showing the design loading (Health Department Guidelines), the
loading at failure, and the loading after recovery.The data includes Flow, (GPD), (Flows per square foot), BOD5,
TPPD of BOD5, (Total pounds per day per square foot), TSS, (Total
Suspended Solids), FOG, (Fats, Oils, Grease), pH, Temperature, and DO,
(Dissolved Oxygen).

One Aerobic Treatment Unit (ATU)
product line was selected for the study, which consisted of a residential model
and a commercial model.Both models
maintained similar features including; 1) control of system flow, 2) the
reduction BOD5 by a percentage, 3), the discharge of a very high DO
with an pH near seven, and 4), and discharge aerobic microorganisms.

*William L Stuth, Aqua Test, Inc., PO Box 1116 Black Diamond
WA98010

Matt M. Lee, Aqua
Test, Inc., PO Box 1116 Black Diamond WA98010

CASE STUDIES

Case Study #1:

In April 1996 Aqua Test, Inc., a
monitoring and maintenance company was employed to evaluate a failing onsite
septic system serving a three-bedroom home.The system was six years old with a household consisting of two adults
and two teenagers.The original septic
system design was based on 360 GPD with a loading rate of .8 g/ft2
or 450 ft2 of drainfield.The septic system consisted of a 1000-gal, two-compartment tank and 225
ft., two-foot wide serial distribution, gravity drainfield.The lengths of drainfield lines starting
adjacent to the septic tank are as follows: 28, 30, 70, and 97 respectively
(Fig. 1).

Figure 1.Case Study #1.Biologically Failed Drainfield.

At the time of the first
inspection, all drainfield lines were flooded and the lowest line in elevation
was surfacing.The owner reported the
onsite system had been pumped out three times since installation, at two, three
and a half, and five years.Each time
the system was pumped because sewage had backed up into the home.

The system evaluation was
initiated with a soil investigation that consisted of describing the soil
profile between drainfield lines one and two to a depth of 64.The results of which indicate the site soils
exhibit the characteristics of Loamy Sand to a depth of 60, verifying the
.8-loading rate called for in the original design, followed by Hardpan or
Glacial Till between 60 and 64.

The system evaluation concluded
with an analysis of the wastewater characteristics including BOD5,
TSS, FOG, pH, Temperature, and DO.An
effluent sample was collected from the outlet of the septic tank on
5/17/96.The results are shown in Table
1.Water district records indicated the
flows to be 300 GPD.The Washington
State Regulation defines the maximum residential BOD5 waste strength
as 230 mg/L and no clear definition for TSS and Fats, Oil and Grease.Aqua Test, Inc. maintains an extensive
database for residential waste strengths analyzed in Washington.From this data base the maximum residential
waste strength for TSS and Fats, Oil and Grease are 76 mg/L and 25 mg/L respectively.Based upon the frequency of pumping and the
analytical testing results, Aqua Test, Inc. and the Health Regulator concluded
that organic overloading, which resulted in the formation of a heavy biomat,
was the most probable cause for the drainfield failure.After review of the site assessments and
conclusions, the construction of additional drainfield would be an unwise
repair.The addition to the drainfield
without a simultaneous change in the waste stream would consequently result in
another failure within a relatively short period of time.However recovering the existing system
utilizing an ATU and adding 100 ft. of drainfield would provide an adequate
long-term solution.Table 1 summarizes
the waste strengths and loading rates for the three modes of operation as
follows: the design, the failure, and the recovery.The loading rates in the table are reflective of the total
drainfield area receiving effluent.

Table 1.Summary Table.

Mode

Flow

(gpd)

Unit flow

(gal/ft2
)

BOD5

(mg/l)

BOD5 # per day

BOD5 # per ft2 per day

TSS

(mg/l)

FOG

(mg/l)

pH

Temp

(C)

DO

(mg/L)

Design

360

0.8

230

.69

.0015

76

25

NA

NA

NA

Failure

300

0.66

340

.85

.0019

24.7

57.7

6.6

18°

.3

Recovery

313

5.59

61.7

.16

.0029

35

26

7.6

20°

5.5

Available Area

313

.48

61.7

.16

.00024

The repair was initiated through
pumping the septic tank and the first line of the drainfield, followed by the
installation of an ATU in the second compartment (outlet) of the 1000 gal.
septic tank and 100 of drainfield added onto the end of the drainfield
lines.After 30 days, only a small
amount of ponding was evidenced in each of the drainfield lines.Within 90 days of installation, the only
line showing evidence of ponding was the first line, which at this time is
still the only line that shows any trace of ponding.The total flow from the home, now averaging 313 GPD,is being
completely absorbed into the first 28 of drainfield.

Case #2:

In early February 1996, Aqua
Test, Inc. was asked to investigate a failing onsite mound system located at a
three-bedroom home built in 1990.The
owner reported that the system had first surfaced in 1992, when the home was
two years old.For this particular
residence, the family consisted of two adults and two small children.The onsite system had been designed for 360
GPD with a loading rate of 1.2 g/ft^2 or 300 ft2of
disposal bed.The system consisted of a
1000 gal. two-compartment septic tank, a 250 gal. pump tank, a 1/3-hp. float
controlled pump, and a mound (Fig.2).

Figure 2.Case Study #2.Failed Mound.

The investigation of this system
included characterizing the waste stream and measuring the flows.Effluent samples were collected from the
pump tank on 2/22/96 and results are shown in Table 2.At the time of sample collection, an hour
meter was installed on the pump in the pump tank.Daily readings were recorded and after a one-month period, the
flows were averaging 196 GPD.Further
investigation showed 52 gal. were being pumped in a two-minute cycle.This is exactly half of what the pump was
discharging at time of installation.The GPM was due to ponding in the gravel bed in the top of the
mound.The amount of effluent surfacing
was determined to be approximately 10 gal. per cycle.This meant that the mound was absorbing roughly 42 gal. per
cycle.Note: Prior to determining
the amount of effluent surfacing, the mound had been allowed to rest for two
hours.A further investigation was
carried out as to the composition of the sand used in the bed of the
mound.A sample of the sand was
collected and subjected to a sieve analysis. The results indicated that the
sand passed the C-33 standards used for classifications in this area.Table 2 below summarizes the waste strengths
and loading rates for the three modes of operation as follows: the design, the
failure, and the recovery.The loading
rates in the table are reflective of the disposal bed area receiving effluent.

Table 2.Data Summary

Mode

Flow

(gpd)

Unit Flow

(g/ft2 )

BOD5

(mg/l)

BOD5 # per day

BOD5 # per ft2 per day

TSS

(mg/l)

FOG

(mg/l)

pH

Temp

(C)

DO

(mg/L)

Design

360

1.2

230

.69

.0023

76

25

NA

NA

NA

Failure

196

.65

262

.43

.0014

36

61.

7.4

16°

.2

Recovery

319

.94

63.5

.17

.00056

31.3

10.1

7.7

23°

6.3

Aqua Test, Inc. and the health
regulator reviewed the information collected to date and concluded that organic
overloading, which resulted in the formation of a heavy biomat, was the most
probable cause of the mound failure.

At this point, Aqua Test, Inc.
recommended a phased approach to correct the failing system.Phase I would be the recovery of the
existing onsite system and Phase II would be the installation of additional
drainfield.Phase I included the
installation of an ATU in the outlet of the septic tank to recover the
partially failing mound, and monitoring of the system for 30 days.If the surfacing stopped, the drainfield
would not be installed.In the event
that the mound continued to surface, Phase II, the proposed drainfield
addition, would be installed and a sufficient amount of the flow would be
directed to the drainfield.This would
prevent the mound from surfacing.All
parties agreed that a phased approach would be implemented.Phase I commenced with the pumping of the
septic tank and the gravel bed in the mound, followed by the installation of an
ATU on 4/16/96.On 4/23/96, effluent
was no longer surfacing.The system has
been continuously monitored biannually.Evidence of surfacing has never been detected at this site since the ATU
installation.As a result, Phase II
has never been implemented.

Case #3:

Case #3 involves a café serving a
menu of typical American dishes.The
restaurant is open seven days a week, serving lunch and diner over a ten-hour
period. The seating capacity is 45 corresponding to 150 meals per day on average.

There are no historical records
available prior to the installation of a new onsite system in late 1986.The design flow used to size the site was
1100 GPD.The new system consisted of
two 1000 gal. septic tanks, one serving the black water, the second serving the
gray water.A common line conveys the
flow from the tanks to the drainfield.The drainfield is a gravity serial distribution system that uses drop
boxes between the lines.There are
eight trenches, 3 wide and 65 long for a total of 1,560 ft2
corresponding to a loading rate .70 g/ft^2 (Fig 3).

Figure 3.Case Study #3 Commercial System.

In1988, within two years of the installation, the gravity
drainfield failed.The failed
drainfield was overgrown with blackberries, which hid the failure from the
public eye.In 1991, citizens were
filing complaints with the Health Department regarding the odors coming from
the failing onsite system.To determine
the waste strength of the effluent, the Health Department collected samples
from the flow as it entered the first drop box.The results are shown in Table 3.The Health Department notified the owner that the problem needed
to be corrected within 30 days or the facility would be closed.Table 3 summarizes the waste strengths and
loading rates for the three modes of operation as follows: the design, the
failure, and the recovery.The loading
rates in the table are reflective of the total drainfield area receiving
effluent.

Table 3.Test Results From Sample Collected By The
Health Department In 1991.

Mode

Flow

(gpd)

Unit Flow

(g/ft2)

BOD5

(mg/l)

BOD5 # per day

BOD5 # per ft2 per day

TSS

(mg/l)

FOG

(mg/l)

pH

Temp

(C)

DO

(mg/L)

Design

1100

.64

230

2.11

.0014

76

25

NA

NA

NA

Failure

1013

.64

4900

41.39

.027

1700

130

NA

NA

NA

Recovery

891

1.14

130.74

.97

.0012

99.4

13.84

7.03

17

.96

Aqua Test, Inc. was provided the
information gathered by the Health Department, shown in Table 3, and employed
to design a system that would comply with Health Department requirements.A drainfield investigation was conducted to
determine the soil profile, soil depth, and depth of drainfield.The soil log report depicts the soil as
Loamy Sand from zero to 52 and Glacial Till or Hard Pan to 60.The existing drainfield was installed in the
top 24, which met local regulations for drainfield separation.With the information collected to date, Aqua
Test, Inc. again recommended a phased approach.Phase I consisted of the following:

·ATU would be installed to reduce the waste strength of
the gray water to residential levels.

·The existing drainfield would be vacuumed out.

·The system would be restarted and monitored weekly for
a two-month period.

If it were found during the
monitoring period that drainfield lines seven and eight were ponded, or sewage surfaced,
Phase II would be implemented.Phase II
would be the replacement of the entire existing drainfield with a new
drainfield.Since the startup of the
ATU to this date, 8/20/00, there have never been more than four lines
ponded.The average of ten samples
collected in the 30 months since the Phase I implementation is shown in Table
3.All of these samples were collected
from the flow as it enters the first drop box, the same location as the Health
Department collected the original sample (Fig.3). Note:The routine
pumping of this system has included the vacuuming out of drainfield lines one
and two.

Summary

As has been demonstrated by the
case studies presented in this paper, before attempting recovery of any onsite
septic system using aerobic effluent, you must first establish whether or not
an excessive biomat is truly the problem.If the biomat is verified to be the cause of the problem, you must
evaluate the extent of the clogging.If
the onsite disposal system will not hydraulically accept any flow, it may not
be recoverable without adding additional disposal area.In reviewing the records of many recovered
systems, clearly the fastest recoveries occur when the treatment or disposal
system is vacuumed out before the aerobic effluent is introduced into the
system.The question to ask ourselves
is what are the differences between aerobic and anaerobic effluent as they
relate to the State Design Guidelines?